The field of the invention is nuclear magnetic resonance imaging (MRI) or spectroscopy (MRS) methods and systems. More particularly, the invention relates to a phantom that may be used to study the effectiveness of MRI and MRS methods and materials.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Phantoms are devices that are placed in the bore of an MRI system to test or calibrate its operation. Phantoms may be made of materials having known magnetic resonance properties or they may contain cavities filled with such materials. The MRI system is operated with the phantom in place to produce a spectrum or an image from which proper operation of the MRI system may be determined. The shape and size of the phantom or its cavities may be designed to measure magnetic field strength or field homogeneity and it may be used in combination with a procedure that enables calibration or trimming of such fields. Phantoms used in this manner are exemplified by those disclosed in U.S. Pat. No. 5,036,280; and published U.S. Pat. Appln. No. 2003/0086535.
Phantoms are also designed to model anatomical structures so that imaging or spectroscopy methods can be developed which accurately depict or represent such structures. As described in U.S. Pat. No. 6,205,871, for example, vascular structures are modeled with a phantom in order to test the efficacy of magnetic resonance angiography methods, and a phantom is disclosed in published U.S. Pat. Appln. No. 2006/0195030 that uses fiber bundles to create phantoms for use in testing diffusion tensor imaging (DTI) methods.
Manufacturing methods enable phantoms to be constructed with subresolvable regions. Three-dimensional fabrication methods such as printing on a thin film sheet or substrate using photo lithography, electrostatic xerographic printing or etching as disclosed in U.S. Pat. No. 6,720,766 may be used to produce cavities and fluid passages in the phantom that are smaller than the voxel size of a high resolution MR image. This technology is used in prior phantoms to model a specific anatomic structure or to achieve a specific effect.